First of all, I would like to say two things. First, I am not a Thor fan and most of what I say regarding Mjolnir comes from this site's wiki or comic books I have read such as the Avengers that do not focus on Thor. Second, I am a Hulk fan because I like who he is and the conflict of his character, not because he is so powerful; I am writing this to set the record straight.
Seeing as Red-Hulk was capable of holding Mjolnir in space it is safe to assume that it works by amplifying the pull of gravity. It means that it can be over powered, there is gravity in space so it would have moved towards either Red-Hulk or Thor had Rulk not over powered it and used it against Thor. Also, it must work be amplifying only enough to equal the force trying to lift it (otherwise it would smash through tables and create large cracks the in the ground). The amount of gravity it can amplify will be limited by Odin's power as it was he who cast the worthiness spell.
Two of Hulk's feats that show he could probably supply this force are: being equal to the Sentry in a fight - Sentry has been described as near omnipotent and has the power of a million exploding suns which is a lot of Newtons of force, Hulk also lifted a 150 billion ton which (according to a conversion rate I found on the internet) is 1471005000000000 N (provided Battleworld has the same gravitational pull as earth, it probably didn't but it would of had a similar size because I can't remember anyone mentioning it), finally, the Beyonder stated that Hulk was infinitely strong.
I'm pretty sure that makes him strong enough to lift Mjolnir but obviously only after a hell of a lot of anger.